In the drilling of oil and gas wells, a mud motor may be used for drilling tasks such as straight hole, horizontal, directional and short radius drilling. Mud motors are connected to the drill string to rotate and steer the drill bit. Mud motors typically include a power section and a bearing assembly. Rotation is provided by the power section that may be a positive displacement motor driven by drilling fluid (mud) circulation. The bearing assembly takes up the axial and radial loads imparted on the drill string during drilling.
The power section of conventional mud motors has a transmission housing, a stator secured within the transmission housing, and a rotor rotatable within the stator. The stator may have a helically contoured inner surface. The rotor may have a helically contoured outer surface. Together, they define a positive displacement fluid pump having a helically shaped progressive cavity.
The bearing assembly of typical mud motors is threadedly connected to the power section. The bearing assembly includes a bearing housing, a rotatable drive shaft secured within the bearing housing for coupling the rotor to the drill bit, and a transmission shaft for connecting the rotor to the drive shaft. The drive shaft extends downwardly through the bearing housing and is supported within the bearing housing by multiple sets of bearings. The drive shaft is fitted with the drill bit at its lower end.
Drilling fluid or mud is pumped from the well surface through the drill string or drill pipe to the mud motor. The drilling fluid or mud flows through the cavity formed between the rotor and the stator, through the portion of the transmission housing surrounding the transmission shaft, into the inlet ports provided on the drive shaft, through the drive shaft, and out through the outlet port to flush cuttings from the borehole and cool the drill bit. The drilling fluid drives the rotor to rotate within the stator, with the rotor orbiting around the inner surface of the stator. The transmission shaft transmits the rotational movements of rotor to the drive shaft. The drive shaft rotates concentrically within the bearing housing to drive the drill bit.
If the drill bit is not in contact with the bottom of the borehole, the high pressure drilling fluid applied through the drill pipe to the mud motor exerts a downwardly-directed axial thrust load to the rotor, transmission shaft, drive shaft and drill bit relative to the bearing housing. This is referred to as an off-bottom thrust load. When the drill bit is brought into contact with the bottom of the borehole, the weight of the entire drill string is imposed on the drill bit. As a result, an upwardly-directed axial thrust load is exerted on the drill bit and drive shaft. This is referred to as an on-bottom thrust load.
Conventional mud motors use a combination of radial bearings and thrust bearings in order to restrict the radial and axial movement of the motor's drive shaft. A cross-sectional view of a typical prior art bearing assembly 1 is shown in FIGS. 1A and 1B. Conventional bearing assembly 1 contains three sets of bearings: upper radial bearings 2; thrust or axial bearings 3; and lower radial bearings 4.
Common radial bearings used in downhole drilling applications are journal bearings, also known as sleeve bearings. A journal bearing is formed from a plain cylindrical sleeve that carries a rotating shaft. Sometimes, journal bearings are also referred to as fluid film bearings because of the presence of a thin film of lubricant formed between the cylindrical sleeve and the rotating shaft. The coefficient of friction experienced by the rotating shaft is dependent, in large part, on whether a fluid film is fully developed. In essence, a fully developed fluid film creates a hydrodynamic pressure sufficient to float the shaft and its respective load relative to the sleeve or journal. The result of a fully developed fluid film is that there is no physical contact between the rotating shaft and the journal during operation. Proper development of a fluid film is generally dependent on adequate lubrication of the bearing journal.
Rolling element-type bearings, such as ball rollers or angular contact rollers, are often used as thrust bearings. U.S. Pat. No. 5,074,681 to Turner et al. discloses an example of ball rollers. U.S. Pat. No. 5,248,204 to Livingston et al. discloses an example of angular contact rollers. Typically, these rolling element-type bearings are lubricated by the drilling fluid (mud) or by clean oil when encased in a sealed oil chamber. Rolling element-type bearings are not tolerant of abrasives and thus wear quickly when exposed to mud lubrication. Once wear occurs, loads between the individual balls become uneven and wear rates accelerate. For the oil-lubricated bearings, once the seals fail, wear occurs in a similar way. Due to the high loads, pressure, and abrasive conditions, bearing life is typically only several hundred hours.
Another type of thrust bearing used in downhole drilling motors is a hydrodynamic or sliding surface type. U.S. Pat. No. 4,560,014 to Geczy discloses an example of this hydrodynamic bearing type, which uses rigidly mounted pads manufactured of industrial diamond. The diamond pads are mud-lubricated and slide against each other. These bearings, however, are extremely expensive and only marginally increase service life.
Bearings in the bearing assembly are exposed to harsh conditions. The bearings are subjected to extreme forces and loads. The bearings also encounter abrasives contained within the drilling mud or other lubricants. Bearings have a limited service life due to operational conditions, regardless of the type of thrust bearings used. Failure of the bearings often results in the thrust load being absorbed by the bearing housing as it contacts the drill shaft. Such metal-on-metal contact causes degradation of the bearing housing. Therefore, when the bearings fail, the entire drill string must be removed from the wellbore so that the mud motor can be replaced. The cost to repair and maintain conventional mud motors is significant. Typical mud motors contain numerous components requiring a skilled technician to undertake the repairs. The mud motors may be out of service for extended periods due to the number of components that must be examined or replaced.